Various structures are known that enable such a function to be implemented, in particular waveguide structures such as those described in the work entitled Radar Handbook, 1970, edited by M. Skolnik, and in particular chapter 13 entitled Frequency-Scanned Arrays by Irving W. Hammer which describes, in particular, slot arrays and structures having folded radiating elements enabling such electronic scanning to be implemented by frequency variation.
French patent publication FR-A-2 535 120 in the name of the present Applicant also describes a frequency-sensitive reflector element which, when placed in front of a wave launcher such as a transmitter horn serves to reflect the incident wave in a direction that varies as a function of the frequency of said wave.
However, all of these devices suffer from various common drawbacks, namely:
their scanning ability (i.e., the amplitude of the angular variation in the direction of the main lobe as a function of the maximum relative frequency variation) is generally very limited, and insufficient in numerous applications; PA1 their structure is always complex both from the mechanical point of view and from the radio point of view, thereby making design and manufacture difficult, and therefore expensive; PA1 these complex structures are generally massive and voluminous, which makes them ill-suited for use as satellite antennas; and PA1 the shapes of the radiation patterns produced are such that on changing frequency, the degree of overlap between two successive beams (i.e., the level in a direction halfway between the main transmission directions of two successive beams) is generally relatively low, thereby making it difficult to obtain continuous coverage of a given geographical area. PA1 (1) high power can be radiated at high efficiency; PA1 (2) polarization linearity is maintained; PA1 (3) circular polarization may optionally be used; PA1 (4) the structure is robust, and suitable for withstanding the severe stresses of the space environment; and PA1 (5) maximum insensitivity exists to temperature variation, which is particularly useful given the very large amplitude temperature cycles encountered in the space.
An object of the invention is to provide a frequency-scanned antenna which remedies all of these drawbacks, thereby making it entirely suitable for use as a satellite antenna, in particular as an antenna for satellite communication.
It is shown that from the mechanical point of view, the structure of the antenna of the invention is simultaneously simple, compact, and lightweight, all of which characteristics are particularly desirable for use on a satellite.
It is also shown that the scanning ability of the proposed structure as a function of frequency is highly sensitive to frequency, i.e., a relatively large scanning amplitude is obtained for a small variation in frequency.
This characteristic is particularly advantageous since the permitted frequency excursion is generally limited by the specific characteristics of the transmitter by microwave bandwidth allocations, e.g., in the 30/20 GHz bands used for satellite communications where bandwidth is typically about .+-.2.5% around the center frequency. With frequency excursion limited in this way, it is desirable to be able to cover as wide a geographical area as possible while remaining within these frequency limits. This is a characteristic which the present invention specifically provides, together with the possibility of easily establishing by construction the most appropriate frequency sensitivity given the desired geographical coverage, merely by selecting simple geometric parameters.
It is also shown that the antenna of the invention is entirely compatible with various common constraints such as: